The invention pertains to a resin composition comprising a styrene maleic anhydride copolymer as a cross-linking agent (curing agent), an epoxy resin, and a co-cross-linking agent.
The use of cross-linking agents for epoxy resin is described in BE 627,887. This patent also discloses a proposal to use copolymers of maleic anhydride and styrene (SMA) as cross-linking agent for epoxy resin. A drawback to such epoxy resin compositions is that they have low Tg and low thermal stability, rendering them unsuitable for use in prepregs, which are applied in laminates for printed wiring boards (PWBs).
The resin generally used in electrolaminates is an epoxy resin. The present practical standard is the FR4-laminate, which is based on a brominated epoxy resin prepared from a diglycidyl ether of bisphenol-A and tetrabromo-bisphenol-A, dicyanodiamide as curing agent, an organic solvent, and an accelerator. The drawback to such an epoxy resin is its low Tg (110-130° C.), while in addition the dicyanodiamide has a tendency to crystallize in the resin and the prepreg made therefrom.
An improvement has been sought in the preparation of an interpenetrating polymeric network (IPN). Such resin compositions are known from EP 413,386. This document relates to IPNs having very favorable properties, in particular for use in the electronics industry. This is the case when the cross-linking agent used for the epoxy resin is a polybrominated phenol. In actual practice, the embodiment using anhydride cross-linking agents proves unsatisfactory. Notably, the Tg obtained is too low, and the electrical properties and the prepreg stability also leave room for improvement.
In addition, it is desired that the use of inexpensive difunctional epoxy resins should give thermal properties which are of the same standard as can be obtained using the multifunctional epoxy resins preferably employed in EP 413,386. Resins based on multifunctional epoxy compounds have been described in WO 85/03515 and WO 86/02085.
Other publications describing epoxy resin compositions employing anhydrides as cross-linking agent for the epoxy resin are U.S. Pat. No. 2,707,177; DE 3,521,506; GB 994,484; and EP 417,837. This last patent specification teaches the use of ethylenically unsaturated anhydrides, such as maleic anhydride, where the anhydride not only cross-links the epoxy resin but also takes part in the forming of the network.
A solution to the above-mentioned problems has been suggested in WO 96/07683, which discloses a resin composition where the carboxylic anhydride is a copolymer of an ethylenically unsaturated anhydride and a vinyl compound. In such a copolymer the ethylenically unsaturated portion of the anhydride is incorporated into the backbone. The carboxylic anhydride groups remain intact, and they are available as functional groups for cross-linking the epoxy resin. More specifically, those resin compositions contain triallyl cyanurate (TAC) as allyl polymerizing agent. In this type of resin TAC is necessary to obtain compositions with high Tg and acceptable thermal stability, which can be applied in prepregs.
Resin compositions which comprise dicyandiamide as co-cross-linking agent have been disclosed in DE 38,39,105. According to this document, dicyandiamide is an essential constituent of the resin composition. Dicyandiamide, however, has the disadvantage that it only dissolves in toxic and expensive solvents, and it would be an advantage to find suitable co-cross-linking agents, devoid of the disadvantages of dicyandiamide.
Epoxy resin compositions containing low-molecular weight copolymers of α-methylstyrene and maleic anhydride have been disclosed in U.S. Pat. No. 4,042,550. Such compositions are unsuitable for the manufacture of PWBs.
Prepregs are widely employed in the manufacture of laminates for the electronics industry, in particular for printed wiring boards. Such manufacture involves impregnating a supporting or reinforcing fabric with a resin, followed by partial curing of said resin. Such impregnated fabric is commonly referred to as prepreg. Manufacturing a printed wiring board involves laminating one or more layers of prepreg with, say, one or more layers of copper.
Processing prepregs into boards usually involves their being cut down to size and laminated. Both these process steps make stringent demands on the resin with which the fabric is impregnated. For instance, the partially cured resin has to have sufficient sturdiness and a high viscosity, yet it must be sufficiently sticky and liquid to give good adhesion when laminated, and hence good interlaminar strength. The resin may not be too highly reactive, since this will render the required partial curing impossible.
In this connection resin compositions where the epoxy resin is cross-linked with an anhydride-containing copolymer have the drawback of being too brittle to be processed as prepregs. For instance, it proves impossible to cut up such prepregs without a portion of the resin blowing about in the form of a large quantity of dry dust. This is sometimes called a “mushroom effect,” after mushroom spores blowing about.
It has now been found that in contrast to the previous solutions for obtaining suitable polymers for use in prepregs, IPNs are not necessary, and that epoxy resins free from TAC can be prepared having high Tg and/or improved thermal stability.
On the one hand, the invention has the object to enhance the thermal and electrical properties of resin compositions based on epoxy resin cross-linked with styrene maleic anhydride copolymer (SMA). On the other hand, the invention envisages resin compositions based on difunctional epoxy resin which have thermal and electrical properties comparable to IPNs the resin composition of which is based on multifunctional epoxy compounds. Furthermore, the invention aims to provide resin compositions where the problem of brittleness, which occurs when SMA is used as epoxy cross-linking agent, can be prevented.